I've implemented the above in userspace for my own experimentation (not yet ready for public release, but maybe soon.) Nearly everything I've described above can be implemented with relatively straight forward SQL queries and SQLites performance has been everything I could have asked from it (in the neighborhood of ten thousand tags and hundreds of thousands of files.)

Getting file tagging into the kernel level to replace directory hierarchies would be a huge paradigm shift, a very dramatic departure from Unix. To be honest I'm not sure whether or not getting such a system into a kernel would be appropriate or not. Traditional hierarchical file management seems more than sufficient for "system files". But I'm really interested in replacing hierarchical file management from the users' perspective. More or less, put ~/Documents, ~/Downloads, ~/Desktop, etc under control of the tagging system but leave the rest of the system as-is. At least for the proof of concept.

I think a demonstration system could be implemented as a custom desktop environment running on regular old linux, where the open and save dialogs of GUI applications have been replaced with tagging/querying windows. Instead of the user clicking through a few directories to find a file to open or find a directory to save something, they would instead click or type in tags to add or query. The GUI file manager would likewise be replaced with the GUI frontend to the tagging system.